Football as a national sport provides its fair share of agony and ecstasy on the pitch.

But fans of the beautiful game also like nothing more than seeing their beloved team do great business off the park.

And none more so when it comes to buying and selling players - especially for a handsome profit.

The current big-money target is Celtic striker Moussa Dembele who is reportedly attracting a bid of £30m plus - after being bought for a mere £500k.

We look at the top ten most profitable signings in Scottish football.

(Disclaimer: this does NOT include players from clubs' youth systems like Aidan McGeady, Alan Hutton and Craig Gordon).

Celtic: Victor Wanyama

Bought for £900k from Beerschot AC, sold for £12.5m to Southampton (£11.6m profit)

Wanyama became the first Kenyan to play in Scotland after Celtic finished a year-long chase when they signed him in 2011.

Strong and powerful in the tackle, he proved to be a revelation in the Scottish Premiership, making over 60 appearances in a two-year spell, including scoring the opening goal in Celtic’s famous Champions league win over Barcelona.

He departed for a hefty sum to Southampton and now plies his trade at Tottenham.

Celtic: Virgil Van Dijk

Bought for £2.6m from Groningen, sold for £11.5m to Southampton (£8.9m profit)

Celtic unearthed a gem of a defender when they signed Van Dijk from Dutch football in 2013.

Despite some initial doubts from many pundits, Van Dijk seemed to breeze through every domestic game he played, clearly possessing a quality a cut above others in the Scottish league.

It was no surprise when the English Premier League came calling. And Celtic could net a nice sell-on fee in the near future, with moves to Chelsea and Manchester City rumoured to be in the pipeline.

Celtic: Fraser Forster

Bought for £2m from Newcastle, sold for £10m to Southampton (£8m profit)

After two years of loan deals at the Parkhead club, Celtic convinced Newcastle United to part with Fraser Forster on a permanent basis in 2012.

After previously being touted for many years as a future England keeper, it took getting regular game time in Glasgow for this potential to shine through, breaking a Scottish league record that had stood for 40 years for the longest amount of time without conceding a goal, going unbeaten for 1,256 minutes.

Again spotting potential in the Scottish market, Southampton would come knocking in 2014.

Forster earned a place in the England squad for both the 2014 World Cup and Euro 2016 as back-up keeper to Joe Hart.

Rangers: Jean-Alain Boumsong

Bought on a free from Auxerre, sold for £8m to Newcastle (£8m profit)

In 2004, Rangers beat off interest from Liverpool to land Boumsong from French high flyers Auxerre.

A centre back who appeared to process a great reading of the game, Boumsong adapted well to Scottish football, leading to a move to Newcastle United after just six months in Scotland for £8m.

In England though, his form collapsed dramatically and he would go on to be labelled a flop. Spells at Juventus and Lyon would follow, though he would never recapture the form he showed at Rangers. As of writing, he has been without a club since 2013.

Rangers: Carlos Cuellar

Bought for £2.37m from Osasuna, sold for £7.8m to Aston Villa (£5.43m profit)

After impressing then Rangers boss Walter Smith in a UEFA cup tie the previous campaign, Smith paid £2.37m to bring Cuellar to Ibrox in 2007 on a four year deal.

In what turned out to be his sole season in Glasgow, Cuellar impressed hugely in partnership with veteran defender David Weir as Rangers challenged in four competitions, winning both the Scottish Premier League Player of the Year and Scottish Football Writers Player of the Year.

After being advised to leave Scotland in order to get opportunities at international level, Cuellar moved to Aston Villa, where most of his appearances would come out of position at right back, preventing him from producing the same level of form that made him such a success in Scotland.

A return to Rangers nearly materialised in 2011, but fell through and Cuellar is now plying his trade in Israel with Maccabi Petah Tikva.

Celtic: Ki Sung-Yueng

Bought for £2.1m from FC Seoul, sold for £6m to Swansea (£3.9m profit)

A highly thought of young player in Asia, twice named in the K League Best XI and a one time winner of Asian young player of the year, it took £2.1m to sway Ki to Scotland as opposed to the lights and glam of Portsmouth of the English Premiership in 2009.

After a mixed start he would truly come into his own in 2011-12, which earned him a move to Swansea City for a then club record fee of £6m.

In Wales, he featured in the Swans' League Cup-winning side of 2013. He remains at Swansea to this day, with a year long spell at Sunderland on loan sandwiched in the middle.

Celtic: Stiliyan Petrov

Bought for £2.8m from CSKA Sofia, sold for £6.5m to Aston Villa (£3.7m profit)

Signed by John Barnes, Petrov was just 20 years old when he arrived in Glasgow in 1999.

Despite featuring regularly in his first season, Petrov struggled greatly with homesickness and being played out of position.

After improving his English by working on a friend’s burger van, Petrov would go on to become a key player in Martin O’Neill’s conquering Celtic team of the early noughties, scoring 55 goals in a seven year spell where he developed into one of the leagues top box to box midfielders.

He would leave Scotland in 2006 to reunite with O’Neill at Aston Villa for £6.5m, where after a couple of years of patchy form, Petrov would eventually win over the support after adapting his style to that of a defensive midfielder.

He was sadly forced to retire in 2013 after being diagnosed with acute leukaemia. He recovered and has currently been given the all-clear.

Rangers: Giovanni Van Bronckhorst

Bought for £5.5m from Feyernoord, sold for £8.5m to Arsenal (£3m profit)

One of the Dutch revolution of players who arrived at Ibrox alongside new manager Dick Advocaat in 1998.

Over a three-year spell in Glasgow, he would score 22 goals, showing his skill and subtlety as a playmaker, earning an £8.5m move to English high flyers Arsenal.

A combination of injuries and poor form meant he struggled in London, but he would soon reinvent himself as a left-back at Barcelona, winning the Champions League in 2006, before captaining his country to the World Cup final in 2010.

He is now manager of Dutch side Feyenoord, where he has led them to the top of the Eredivisie.

Celtic: Scott McDonald

Bought for £700k from Motherwell, sold for £3.5m to Middlesbrough (£2.8m profit)

Not many players sign for Celtic after being endeared by the support of Rangers, but that was the situation Scott McDonald had when he arrived at Celtic in 2007, after scoring a double against them on the last day of the season two years prior to win Rangers the league title.

McDonald became a firm favourite a Parkhead scoring 65 goals in a three year spell with the club.

After a dip in form in his final season in Glasgow, McDonald choose to reunite with the manager who signed him for Celtic, Gordon Strachan in 2010 at recently relegated English side Middlesbrough.

Now heading towards the twilight of his career, he rejoined Motherwell in 2014.

Celtic: Gary Hooper

Bought for £2.4m from Scunthorpe, sold for £5m to Norwich (£2.6m profit)

After letting Scott McDonald leave in 2010, Celtic would replace him that same year with arguably a better finisher in Hooper.

In three years he scored over 80 goals, including a superb five goals in 10 meeting with rivals Rangers.

Hungry for success at a higher level, Hooper would join Premier League Norwich in 2013, where he never seemed to be able to tie down a regular place in the team.

He is currently helping Sheffield Wednesday in their charge for promotion to the Premier League via the playoffs.